The greatest rock climber in the world is climbing the greatest rock in the world. Alex Honnold is on El Capitan, free-soloing it – meaning no rope, no one else, just a man alone on a wall. He is at the crux, the most difficult section, known as the boulder problem, the main problem being that it is really, really hard. But he moves gracefully, balletically even: drive up off the left foot into the thumb press, roll two fingers over the thumb, switch feet, left foot out to a bad sloping foothold, switch thumbs, reach out left to a grainy rounded hold before launching into the karate kick … And that is where he slips and falls.

Not 700 metres to his death, though, which is what would have happened if he was on the real El Cap, in Yosemite, California. He is actually in Vauxhall, south London, at a climbing centre where they have tried to recreate the hardest section of the route that Honnold really did climb, alone and without a rope. So this time he has fallen about two and a half metres on to a crash mat, nothing hurt except a little pride. “I’m dead,” he laughs. Hahaha.

Alex Honnold, now 33, has been a legend in the sport for a while, with a rack of insane firsts and nobody-will-evers hanging from his harness (except he doesn’t usually wear one of those). With a goofy grin and a bad haircut, he has been fighting a single-handed battle against gravity, and winning. When, on 3 June 2017, he free-soloed the freerider route on El Capitan, the New York Times described it as “one of the greatest athletic feats of any kind, ever”.

Then the film about that climb – Free Solo – came out, and the world outside the climbing community sat up and took note. It is a brilliant, beautiful film – not just the story of an incredible physical performance (with some of the most buttock-clenchingly tense viewing you’re ever likely to squirm through), but a very human story of a remarkable, beguiling character. Oh, and it’s a love story, too. It has just won the Oscar for best documentary, after winning a Bafta earlier in the month.

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Free Solo.

Honnold is in town to promote the film. You can’t keep him away from climbing for long, though, even in the city, and this is where he comes when he is in London. It’s not unlike the climbing wall where he started off in Sacramento, he says. There is a buzz about the place among the men and women who work here – imagine Lionel Messi dropping in to your amateur football club.

They have recreated this section of the boulder problem in his honour. But it’s not quite right: this hold needs to come in a bit, the thumb press presses the wrong way, there shouldn’t be a foothold on the end of the karate kick … He is telling them where everything should be from memory. He knows every millimetre of this section of El Capitan, practised it 40 or 50 times with a rope before attempting it without. That – the meticulous practice and preparation – is the key to not falling. If the recreated boulder problem was exactly the same as the real boulder problem, it wouldn’t have been a problem.

Honnold climbs because he loves it; he grew up doing it. “I like the movement, I like swinging, it all feels kinda playful and fun.” But why without a rope, when the stakes are so much higher? He has obviously been asked the question a thousand times before, but he still seems to think about it. “It’s like when I say that climbing is all about fun; free soloing is sort of the extreme. If you do something for fun all the time, every once in a while you want to have consequences.”

I speak to Jimmy Chin, who filmed a lot of Free Solo and co-directed it with his wife, Elizabeth Chai Vasarhelyi, after a screening of the documentary. A hugely accomplished climber himself, he says he wouldn’t attempt to free-solo a single pitch of El Capitan, but he understands what drove Honnold. “If you had a superpower and you could fly, you would probably do it, right?” he says. “The drive and ambition to do something that pushes you, that you love – it’s hard to put that away and not use it.”

Chin is in no doubt about what it meant to free-solo El Capitan, which he compares to an Olympic gold medal gymnastics floor routine. “But the floor routine is four hours long. And every single move, every second, you have to be performing perfectly, knowing that if you make a single mistake you would die.”

Honnold knew he wasn’t going to fall off El Capitan because of the practice and preparation. What about all the other people mentioned in the film who have died free-soloing – people he knew, such as Ueli Steck? “He died climbing a mountain at 7,000 metres in the snow wearing crampons – it’s a completely different experience,” Honnold says. Another climbing legend, John Bachar? He did die free-soloing, but “with extenuating circumstances. He’d been in this car accident and had nerve damage. He died soloing on something quite easy.”

Dean Potter, a free-soloing pal of Honnold’s who also features in the film, died while Base jumping: jumping off a cliff with a parachute. Honnold wouldn’t do that, “because everyone dies, honestly”. He explains the difference in risk. With Base jumping, you die unless everything works perfectly – the parachute opens, you’re facing the right way when it does etc. With free-soloing, you’re OK unless something goes badly wrong.

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The making of Free Solo.

If any of this talk about death – of people he knew and potentially his own – comes across as unfeeling or callous, it doesn’t seem like that at the time. He just has a matter-of-fact directness that is arresting, but also honest and refreshing. He is not an adrenaline junkie thrill-seeker: he climbs because he loves to climb, not because of the danger of death. A lot of work goes into minimising that danger.

He boulders as we talk, picking his way up, sometimes three metres above me, stretched out, a human bridge between a couple of smears on the wall, then back down at ground level again. I have a go and I can’t even get on to the beginning of the boulder problem; the idea of 600 metres of air below me is ridiculous. I try something easier and Honnold offers advice and encouragement from below. I do a bit of outdoor climbing, but I get scared high up on exposed routes. Keep on doing it until the fear goes away, he tells me.

Eventually, we sit down. It’s freezing, so he puts on a jacket by a well-known outdoor clothing company. It pays him about what a well-paid dentist gets, he says in the film. Since then, it has become more “like a really successful orthodontist”, he admits. Anyway, he is no longer a dirtbagger living in a van, but has a house in Las Vegas. If Vegas seems an unlikely place for him, it has nothing to do with the city itself and everything to do with the fact it is surrounded by some of the best climbing in the country.

He lives with his girlfriend, Sanni McCandless, a life coach who plays a big part in Free Solo. That is the love story bit, although it’s not easy dating a guy who crawls out of bed before dawn to go and hang off cliffs. When he reached the top of El Capitan after three hours 56 minutes (to put it in perspective, it took a German team of professional climbers doing it with ropes four days), with the biggest grin of his life, he called McCandless. “I love you,” he just about managed to say, awkwardly, before ruining it by downgrading it to: “I appreciate you.”

He’s not really an I-love-you kind of guy, is he? “No, no,” he agrees. Nor much of a hugger. That comes into the film, that there wasn’t much hugging in his family growing up. One critic described it as “a cautionary study of what can happen when you don’t hug your children”. But Honnold has taught himself to hug, he says, and he is getting better at it. Can I get one? Sure. We stand up and embrace; there is even a little shoulder- patting going on. “That’s nice, huh?” he says. It is – big strong arms.

The no-huggy thing is part of something else going on in the film. He doesn’t always seem switched on to McCandless’s emotional expectations. His remark that previous girlfriends have accused him of having a personality disorder; his obsessiveness; the extraordinary detail of his preparations and the pages and pages of notes; the fact that he began to climb alone in Yosemite because it was easier than asking people to climb with him; his mum saying his dad had Asperger’s … It is implicit rather than explicit, but you could easily come away from watching Free Solo thinking that Honnold might have a rubber-soled toehold somewhere on the spectrum.

He recognises that people might think that, and that it is a seam in the film, and he is not surprised or in any way offended. He wouldn’t mind if he did have a spectrum disorder, but he doesn’t think he does and picks holes in some of the evidence. He was shy, is shy, doesn’t like talking to strangers much now (my hug is feeling more and more special), but he also wanted to climb on his own – there is a strong tradition of it in Yosemite. The obsessive focus and detail? Any elite climber would and has to do the same.

He says he has no problem knowing what other people are thinking – McCandless, for example. “I can tell she would prefer something different but, you know, do I have to? Is she going to leave me? No. So you’re sort of like, well, I guess it’s not that serious.” Risk assessed, safe to continue, basically. He goes on: “I have no problem knowing what other people are thinking on a rational level; I just don’t always necessarily care. I’m not necessarily compassionate, but I understand.” Again, it might come across as callous, but it’s also brutally frank.

As for his dad (who died when Alex was 18) having Asperger’s, he disputes it. “Mum says that; I think it’s slightly unfounded.” He was never diagnosed. “I would have for sure called him a kind of quirky man, and pretty quiet.”

Is that what Alex is? “Yeah, totally.” He hasn’t sought a professional opinion for himself, although he once did an online test, which he realises isn’t rigorous. The result? “Totally normal.”

As we talk, he is often distracted, looking around or over my shoulder. Not, I think, because he doesn’t like what I’m asking or because he’s finding the interaction awkward, but because there is something more interesting going on. We are, after all, at a climbing centre. It doesn’t matter if it is the centre staff trying out the El Cap bouldering problem (every now and then, there is the sound of body on crash mat – another tragic death, we laugh), or the woman from the PR company, who has never climbed before, attempting the easiest route in the house. “That’s it, keep straightening that leg, trust it, it’s not that high, yes, you can,” he calls up to her.

Does he like teaching? “I like seeing people succeed,” he says. McCandless, he says, is now “pretty fricking good”. She even likes doing it, which is lucky, otherwise they wouldn’t see much of each other. Same if he – they – ever had kids: they would have to climb. “If they didn’t, it would be hard to spend quality time together,” he says.

Speaking of kids, Honnold has to run – to a school where he is giving a talk. First, though, he has to try the boulder problem again. They have taken on board what he said, moved the holds to where they should be, so it’s near as dammit to the real thing. He switches thumbs, grabs the grainy hold, launches into the karate kick, and lands on the other wall – success.

In Free Solo, it is a key moment, the completion of the karate kick, and he turns to the camera with the biggest grin. There are still 300 or so metres of near-vertical granite to scale, a fiendish crack to work his way up. But, for Honnold, that’s a walk in the park – he knows he’s done it and that he’s alive. Today he jumps down, thanks all the climbing centre guys, grabs his jacket and runs for the waiting car.